Perovskite oxide thin film EL element

ABSTRACT

There are provided a perovskite oxide thin film EL element in which a hole transport layer/a light-emitting layer/an electron transport layer comprising a perovskite oxide thin film are formed on a lower electrode, and an upper electrode is formed thereon, and a perovskite oxide thin film EL element that provides red light emission in the vicinity of a wavelength of 610 nm, which is the basis of display making. A perovskite oxide thin film EL element comprising a lower electrode  1  comprising a polished single crystal substrate, an electron transport layer  2  comprising a perovskite oxide thin film, which is a dielectric, formed on the lower electrode  1 , a light-emitting layer  3  comprising a perovskite oxide thin film formed on the electron transport layer  2 , a hole transport layer  4  comprising a perovskite oxide thin film, which is a dielectric, formed on the light-emitting layer  3 , a buffer layer  5  formed on the hole transport layer  4 , and a transparent upper electrode  6  formed on the buffer layer  5.

TECHNICAL FIELD

The present invention relates to a perovskite oxide thin film ELelement.

BACKGROUND ART

In recent years, EL elements using organic materials and inorganicmaterials, such as rare earth-added ZnS and BaAl₂S₄, for thelight-emitting layer have been developed. But, a problem of such ELelements is that the light emission characteristics are drasticallydegraded by atmospheric exposure. It is indicated that when EL elementsare formed with these materials, wrapping technique and sealingtechnique during assembly are necessary, and their fabrication andmanufacturing line are complicated. Therefore, the product cost isexpensive. Furthermore, the raw materials used for the organic ELlight-emitting layer are more expensive than platinum in terms of costper gram and therefore are a factor of high cost. Further, in inorganicEL, the drive voltage is generally 200 V or more. Therefore, the drivepower supply circuit is large, and slimming down is more difficult thanin organic EL.

A perovskite oxide structure is simple and is of a chemically stablematerial. Therefore, elimination of degradation due to atmosphericexposure and elimination of aged degradation are expected. In recentyears, the research and development of fluorescent materials withperovskite oxide polycrystal powders have been actively performed, andgood fluorescence characteristics have been obtained. On the other hand,for epitaxial thin films, three primary colors of red, green, and blue,which are the basis of display fabrication, have also begun to bedeveloped, and good fluorescence has been obtained, whereas EL providedby electric field application has not been obtained yet. In applicationto displays and the like, an EL element formed by sandwiching alight-emitting layer between dielectric layers on a thin film electrodematerial, with thin films, and fabricating an upper electrode isessential, and the development of a multilayer structure of dielectrics(used as the hole transport layer and the electron transport layer) andthe light-emitting layer with thin films, and the development of an ELelement achieving an interface control technique providing good adhesionbetween the dielectric and the electrode material are supposed to beurgently necessary.

For the fluorescence characteristics of oxide polycrystals, Non-PatentDocument 1 shows that fluorescence characteristics are obtained bysubstitution with Ca, Sr, or Ba in a polycrystal ASnO₃ perovskitestructure. Non-Patent Document 2 shows that blue fluorescence isobtained in a polycrystal Sn, layered perovskite structure. Non-PatentDocument 3 shows fluorescence characteristics when polycrystal CaSnO₃ issubstituted with Tb. Non-Patent Document 4 shows red fluorescencecharacteristics in a polycrystal, layered perovskite Sr_(n+1)TiO_(3n+1)system. Non-Patent Document 5 shows that regarding a SrTiO₃ singlecrystal and thin film, blue white fluorescence occurs due to oxygenloss. Non-Patent Document 6 shows that red fluorescence characteristicsare obtained when polycrystal SrTiO₃ is substituted with a Pr atom.Non-Patent Document 7 shows the red fluorescence characteristics ofpolycrystal, Pr atom-substituted (CaSrBa) TiO₃.

For the fluorescence characteristics of oxide thin films, Non-PatentDocument 8 shows the blue fluorescence characteristics of a thin film,MHfO₃:substituted-Tm. Non-Patent Document 9 shows the fluorescencecharacteristics of a BaTiO₃ thin film substituted with an Er atom.Non-Patent Document 10 shows the red fluorescence characteristics of athin film, CaSrTiO₃:substituted-Pr. Patent Document 1 shows a method formanufacturing a double oxide phosphor thin film in which an inorganicbase material, such as yttrium aluminate, is substituted with a metalion. Patent Document 2 shows a method for manufacturing a thin film thatemits light by the application of a mechanical external force, with amaterial containing a rare earth metal ion or a transition metal ion inan inorganic base material.

For the EL characteristics of oxide thin films, Non-Patent Document 11shows the red light emission characteristics of a thin film EL elementin which Ga₂O₃ is substituted with Eu. Patent Document 3 shows aninorganic-thin film EL element using a ceramic sheet as a transportlayer.

For oxide polycrystals, Patent Document 4 shows the fluorescencecharacteristics of a polycrystal Sn perovskite oxide system. PatentDocument 5 shows that the fluorescence characteristics of red, green,and blue, which are three primary colors of light, are obtained by a Tithin film or a Sn thin film having an oxide perovskite structure. PatentDocument 6 shows regarding an EL element using a Zn₂SiO₄:Mn thin film,which is a non-perovskite structure, for a light-emitting layer, amethod for improving luminance and life.

LIST OF DOCUMENTS

-   Non-Patent Document 1: J. Alloy Compd. Vol. 387, pp L1-4 (2005)-   Non-Patent Document 2: J. Mater. Sci. Lett., Vol. 11, 1330 (1992)-   Non-Patent Document 3: Materials Chemistry and Physics Vol. 93, pp.    129-132 (2005)-   Non-Patent Document 4: J. J. Appl. Phys. Vol. 44, pp. 761-764 (2005)-   Non-Patent Document 5: Nature materials Vol 4, 816 (2005)-   Non-Patent Document 6: Appl. Phy. Lett Vol 78, 655 (2001)-   Non-Patent Document 7: Chem. Mater. Vol 17, 3200 (2005)-   Non-Patent Document 8: Appl. Surf. Sci. Vol 197-198, 402 (2002)-   Non-Patent Document 9: Appl. Phy. Lett Vol 65, 25 (1994)-   Non-Patent Document 10: Appl. Phy. Lett Vol 89, 261915-1 (2006)-   Non-Patent Document 11: Materials Science and Engineering B Vol 146,    252 (2008)-   Patent Document 1: Japanese Patent Laid-Open No. 2003-183646-   Patent Document 2: Japanese Patent Laid-Open No. 11-219601-   Patent Document 3: Japanese Patent Laid-Open No. 2007-157501-   Patent Document 4: Japanese Patent Application No. 2005-322286-   Patent Document 5: Japanese Patent Application No. 2006-190755-   Patent Document 6: Japanese Patent Laid-Open No. 2006-134691

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

It is an object of the present invention to provide a perovskite oxidethin film EL element in which as a basic structure, a hole transportlayer/a light-emitting layer/an electron transport layer comprising aperovskite oxide thin film are formed on a lower electrode, and an upperelectrode is formed thereon.

It is another object of the present invention to provide a perovskiteoxide thin film EL element that provides red light emission in thevicinity of a wavelength of 610 nm, among red, green, and blue, whichare the basis of display making.

Means for Solving the Problems

The present invention uses the following means to solve the aboveproblems.

A first means is a perovskite oxide thin film EL element comprising alower electrode comprising a polished single crystal substrate, anelectron transport layer comprising a perovskite oxide thin film, whichis a dielectric, formed on the lower electrode, a light-emitting layercomprising a perovskite oxide thin film formed on the electron transportlayer, a hole transport layer comprising a perovskite oxide thin film,which is a dielectric, formed on the light-emitting layer, a bufferlayer formed on the hole transport layer, and a transparent upperelectrode formed on the buffer layer.

A second means is a perovskite oxide thin film EL element comprising alower electrode comprising a polished single crystal substrate, a firsttransport layer comprising a perovskite oxide thin film, which is adielectric, formed on the lower electrode, a first light-emitting layercomprising a perovskite oxide thin film formed on the first transportlayer, a second transport layer comprising a perovskite oxide thin film,which is a dielectric, formed on the first light-emitting layer, asecond light-emitting layer comprising a perovskite oxide thin filmformed on the first transport layer formed on the second transportlayer, a third transport layer comprising a perovskite oxide thin film,which is a dielectric, formed on the second light-emitting layer, abuffer layer formed on the third transport layer, and a transparentupper electrode formed on the buffer layer.

A third means is a perovskite oxide thin film EL element comprising alower electrode comprising a polished single crystal substrate ofSrTiO₃(001) in which 0.1% or more of Ti is substituted with Nb, anelectron transport layer comprising a thin film of perovskite oxideSrTiO₃, which is a dielectric, formed on the lower electrode, alight-emitting layer comprising a thin film of perovskite oxide((Ca_(1-x)Sr_(x))_(1-y)Pr_(y))TiO₃: 0≦x≦1 and 0.001≦y≦0.2 formed on theelectron transport layer, a hole transport layer comprising a thin filmof perovskite oxide SrTiO₃, which is a dielectric, formed on thelight-emitting layer, a CeO₂ film buffer layer formed on the holetransport layer, and a transparent upper electrode comprising an ITOfilm formed on the buffer layer.

A fourth means is a perovskite oxide thin film EL element comprising alower electrode comprising a polished single crystal substrate ofSrTiO₃(001) in which 0.1% or more of Ti is substituted with Nb, a firsttransport layer comprising a thin film of perovskite oxide SrTiO₃, whichis a dielectric, formed on the lower electrode, a first light-emittinglayer comprising a thin film of perovskite oxide((Ca_(1-x),Sr_(x))_(1-y)Pr_(y))TiO₃: 0≦x≦1 and 0.001≦y≦0.2 formed on thefirst transport layer, a second transport layer comprising a thin filmof perovskite oxide SrTiO₃, which is a dielectric, formed on the firstlight-emitting layer, a second light-emitting layer comprising a thinfilm of perovskite oxide ((Ca_(1-x)Sr_(x))_(1-y)Pr_(y))TiO₃: 0≦x≦1 and0.001≦y≦0.2 formed on the second transport layer, a third transportlayer comprising a thin film of perovskite oxide SrTiO₃, which is adielectric, formed on the second light-emitting layer, a CeO₂ filmbuffer layer formed on the third transport layer, and a transparentupper electrode comprising an ITO film formed on the buffer layer.

A fifth means is a perovskite oxide thin film EL element comprising apolished single crystal substrate of SrTiO₃(001), a lower electrodecomprising a thin film of SrTiO₃ in which 0.1% or more of Ti issubstituted with Nb, formed on the substrate, an electron transportlayer comprising a thin film of perovskite oxide SrTiO₃, which is adielectric, formed on the lower electrode, a light-emitting layercomprising a thin film of perovskite oxide((Ca_(1-x)Sr_(x))_(1-y)Pr_(y))TiO₃: 0≦x≦1 and 0.001≦y≦0.2 formed on theelectron transport layer, a hole transport layer comprising a thin filmof perovskite oxide SrTiO₃, which is a dielectric, formed on thelight-emitting layer, a CeO₂ film buffer layer formed on the holetransport layer, and a transparent upper electrode comprising an ITOfilm formed on the buffer layer.

A sixth means is a perovskite oxide thin film EL element comprising alower electrode comprising a polished single crystal substrate ofSrTiO₃(001) in which 0.1% or more of Ti is substituted with Nb, anelectron transport layer comprising a thin film of perovskite oxideBaTiO₃, which is a dielectric, formed on the lower electrode, alight-emitting layer comprising a thin film of perovskite oxide((Ca_(1-x)Sr_(x))_(1-y)Pr_(y))TiO₃: 0≦x≦1 and 0.001≦y≦0.2 formed on theelectron transport layer, a hole transport layer comprising a thin filmof perovskite oxide BaTiO₃, which is a dielectric, formed on thelight-emitting layer, a CeO₂ film buffer layer formed on the holetransport layer, and a transparent upper electrode comprising an ITOfilm formed on the buffer layer.

A seventh means is the perovskite oxide thin film EL element in any onemeans of the first to the sixth means, wherein the transport layercomprising perovskite oxide, which is a dielectric, has a latticeconstant in the range of 0.39 nm±0.03 nm.

An eighth means is the perovskite oxide thin film EL element in any onemeans of the first to the seventh means, wherein the perovskite oxidethin film EL element is heat-treated in the range of 900° C. or more and1200° C. or less.

Advantages of the Invention

According to the present invention, a perovskite oxide thin film ELelement with thin films is composed of an electron transport layer and ahole transport layer and a light emitter thin film comprising aninsulator having excellent crystallinity. Therefore, insulator thinfilms having a large dielectric constant value is achieved. Thus, anelectric field is efficiently applied to the light-emitting layer,enabling low voltage drive. Thus, the miniaturization of a drive powersupply circuit system is achieved, and the miniaturization of a displaywith a slim panel would be promoted. Furthermore, a perovskite oxidematerial comprising SrTiO₃ as the base material has excellent chemicalstability. Therefore, even if the perovskite oxide material is exposedto the atmosphere for a long period, no change in composition occurs,and no large change in light emission phenomenon occurs. By using thesefeatures, the degradation of the crystallinity of the sample due toatmospheric exposure is extremely small. Therefore, the degradation oflight emission characteristics is reduced. Thus, complicated wrappingtechnique and the like during assembly are unnecessary, and costreduction is expected.

Furthermore, according to the present invention, a perovskite oxide thinfilm EL element having excellent red light emission characteristics,among three primary colors of red, green, and blue, which are the basicprimary colors of a display, is obtained with perovskite oxide epitaxialthin films, and as a result, it is possible to promote the developmentof electroluminescent (EL) elements with oxide epitaxial thin films.

Furthermore, according to the present invention, Ca, Sr, and Ti atoms,which exist in abundance on earth, and an extremely slight amount ofrare earth element Pr atoms are used, thus being able to contribute tomaterial cost reduction.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing the configuration of a perovskite oxide thinfilm EL element in which a multilayer film of an electron transportlayer 2/a light-emitting layer 3/a hole transport layer 4 comprising athin film is formed on a substrate (lower electrode) 1, and atransparent electrode (upper electrode) 6 is formed thereon via a bufferlayer 5, according to the invention in a first embodiment.

FIG. 2 is a diagram showing an x-ray diffraction pattern during growthat 700° C. according to the invention in the first embodiment.

FIG. 3 is a diagram showing EL characteristics measured for a sampleformed at 700° C. and obtained at an alternating voltage of 25 V at afrequency of 1 kHz according to the invention in the first embodiment.

FIG. 4 is a diagram showing the configuration of a perovskite oxide thinfilm EL element in which a multilayer film of a transport layer 8/alight-emitting layer 9/a transport layer 10/a light-emitting layer 11/atransport layer 120 comprising a thin film is formed on a substrate(lower electrode) 7, and a transparent electrode (upper electrode) 14 isformed thereon via a buffer layer 13, according to the invention in asecond embodiment.

FIG. 5 is a diagram showing an x-ray diffraction pattern during growthat 700° C. according to the invention in the second embodiment.

FIG. 6 is a diagram showing EL characteristics measured for a sampleformed at 700° C. and obtained at an alternating voltage of 40 V at afrequency of 1 kHz according to the invention in the second embodiment.

FIG. 7 is a diagram showing the configuration of a perovskite oxide thinfilm EL element in which a multilayer film of a lower electrode 16/anelectron transport layer 17/a light-emitting layer 18/a hole transportlayer 19 comprising a thin film is formed on a substrate 15, and atransparent electrode (upper electrode) 21 is formed thereon via abuffer layer 20, according to the invention in a third embodiment.

FIG. 8 is a diagram showing an x-ray diffraction pattern during growthat 700° C. according to the invention in the third embodiment.

FIG. 9 is a diagram showing EL characteristics measured for a sampleformed at 700° C. and obtained at an alternating voltage of 25 V at afrequency of 1 kHz according to the invention in the third embodiment.

FIG. 10 is a diagram showing the configuration of a perovskite oxidethin film EL element in which a multilayer film of an electron transportlayer 23/a light-emitting layer 24/a hole transport layer 25 comprisinga thin film is formed on a substrate (lower electrode) 22, and atransparent electrode (upper electrode) 27 is formed thereon via abuffer-layer 26, according to the invention in a fourth embodiment.

FIG. 11 is a diagram showing an x-ray diffraction pattern during growthat 700° C. according to the invention in the fourth embodiment.

FIG. 12 is a diagram showing EL characteristics measured for a sampleformed at 700° C. and obtained at an alternating voltage of 10 V at afrequency of 1 kHz according to the invention in the fourth embodiment.

FIG. 13 is intensity-Phi (deg.) characteristics showing the result ofthe X-ray φ scan of a light-emitting layer, CSTO:Pr(100).

FIG. 14 is a diagram showing the result of the AFM observation of alight-emitting layer.

FIG. 15 is a photograph observed by cross-section TEM, showing thatoriented growth occurs continuously in the vicinity of the boundarybetween a light-emitting layer and an insulator.

FIG. 16 is a photograph in which oriented growth occurring continuouslyin the vicinity of the boundary between another light-emitting layer andinsulator is observed by cross-section TEM.

FIG. 17 is a Nb substitution amount-resistivity characteristic diagram.

Description of Symbols 1 substrate (lower electrode) 2 electrontransport layer 3 light-emitting layer 4 hole transport layer 5 bufferlayer 6 transparent electrode (upper electrode) 7 substrate (lowerelectrode) 8 transport layer 9 light-emitting layer 10 transport layer11 light-emitting layer 12 transport layer 13 buffer layer 14transparent electrode (upper electrode) 15 substrate 16 lower electrode17 electron transport layer 18 light-emitting layer 19 hole transportlayer 20 buffer layer 21 transparent electrode (upper electrode) 22substrate (lower electrode) 23 electron transport layer 24light-emitting layer 25 hole transport layer 26 buffer layer 27transparent electrode (upper electrode)

BEST MODE FOR CARRYING OUT THE INVENTION

The basic structure of a perovskite oxide thin film EL element accordingto the present invention comprises a structure in which a hole transportlayer/a light-emitting layer/an electron transport layer comprising athin film are formed on a lower electrode, and an upper electrode isformed thereon. A dielectric material is used for the hole transportlayer and the electron transport layer, and SrTiO₃ or BaTiO₃, which is aperovskite oxide material, is used by forming a thin film. A redfluorescent material, which is a perovskite oxide thin film,CaSrTiO₃:substituted Pr, is used for the light-emitting layer. Thesematerials have a crystal lattice constant in the vicinity of 3.90 nm,and therefore have excellent lattice matching properties, and can beoriented up to the upper thin film and grown with excellentcrystallinity also as a laminated structure.

In the perovskite oxide thin film EL element of the present invention, amultilayer structure of an oxide light emitter epitaxial thin film anddielectric thin films used as the hole transport layer and the electrontransport layer is formed by a pulse laser deposition method, and aperovskite oxide fluorescent material and a perovskite dielectricmaterial such as SrTiO₃, which are target materials, are formed intothin films by the pulse laser deposition method. An electricallyconductive material comprising, as the base material, SrTiO₃ having goodlattice matching properties with the above materials is used for thelower electrode (substrate), and an ITO material thin film is used forthe upper electrode (transparent electrode). Then, suitable heattreatment is performed to obtain a sufficient withstand voltage, therebyproviding an epitaxial thin film EL element having light emissioncharacteristics in the vicinity of a wavelength of 610 nm (red), whichare the basis of display manufacturing.

Next, a first embodiment of the present invention will be described,using FIGS. 1 to 3.

FIG. 1 is a diagram showing the configuration of a perovskite oxide thinfilm EL element in which a multilayer film of an electron transportlayer 2/a light-emitting layer 3/a hole transport layer 4 comprising athin film is formed on a substrate (lower electrode) 1, and atransparent electrode (upper electrode) 6 is formed thereon via a bufferlayer 5, according to the invention in this embodiment.

In FIG. 1, reference numeral 1 denotes a Nb-substituted STO(100)substrate; reference numeral 2 denotes an electron transport layer, anepitaxial film, SrTiO₃; reference numeral 3 denotes a light-emittinglayer, an epitaxial film, CaSrTiO₃:Pr; reference numeral 4 denotes ahole transport layer, an epitaxial film, SrTiO₃; reference numeral 5denotes a buffer layer, an epitaxial film, CeO₂; and reference numeral 6denotes a transparent electrode, ITO.

The pulse laser deposition method is used for manufacturing theperovskite oxide thin film EL element of the present invention.According to this method, since it is possible to freely select a filmformation atmosphere, it is possible to control the amount of oxygen ina formed thin film, and it is possible to extremely reduce thedegradation of electrical characteristics and fluorescencecharacteristics due to oxygen loss and the like, during the growth of anoxide thin film. The pulse laser deposition method is a method forirradiating a target material comprising an oxide with an ArF(wavelength: 193 nm) excimer laser beam in low pressure oxygen at 1 Torror less, turning the target material into a plasma to form a plume,disposing a heated substrate material on a surface opposed to the targetmaterial, and depositing a thin film. At temperatures of 1000° C. orless, cluster growth is dominant. Therefore, it is possible to depositthe target material with its stoichiometric composition. The laserirradiation frequency is 4 Hz to 8 Hz, and the film formation time is 30minutes to 180 minutes. The distance between the substrate and thetarget is 30 mm to 34 mm. The laser energy is about 1.0 J/cm² to 1.2J/cm². In the present invention, the pulse laser deposition method isused, but a sputtering method, one of other gas phase methods, and filmformation methods with a liquid phase, such as a sol-gel method, may beused for a thin film.

As a typical example, a polished, 1% Nb-substituted SrTiO₃(001) singlecrystal substrate having electrical conductivity is used for thesubstrate (lower electrode) 1. The crystal structure of the substrate 1is tetragonal, and the substrate 1 has a lattice constant of 3,905 nm,and an electrical conductivity of 0.02 Ωcm or less at room temperature,and can be used as the lower electrode 1. Many perovskite oxidematerials have a lattice constant in the vicinity of the numerical valueof the above lattice constant and have good lattice matching propertieswith this substrate material. Therefore, it is possible to grow an oxideepitaxial thin film having excellent crystallinity on this substrate(lower electrode) 1. A polished, 0.1% Nb-substituted SrTiO₃(001) singlecrystal substrate has a resistivity of 0.1 Ωcm or less. Therefore, apolished, 0.1% or more Nb-substituted SrTiO₃(001) single crystalsubstrate can be used.

FIG. 17 is a Nb substitution amount-resistivity characteristic diagram.The characteristic values of the Nb substitution amount-resistivitycharacteristics are shown in the following Table 1.

TABLE 1 Nb concentration Volume resistivity No (Weight %) (Ω · cm) 10.049 0.076 2 0.105 0.032 3 0.480 0.055 4 1.000 0.003

The Nb substitution amount-resistivity characteristics shown in FIG. 17and Table 1 reveal that when the Nb concentration is at least asubstitution concentration of 0.1% or more, the volume resistivity fallswithin a practically unproblematic range of 0.1 Ωcm or less.

A perovskite oxide SrTiO₃ thin film, which is the electron transportlayer 2, is formed on the substrate (lower electrode) 1 at an oxygenpressure of 700 mTorr at substrate temperatures of 700° C. Then, aperovskite oxide Ca_(0.6)Sr_(0.4)TiO₃:0.2% Pr thin film, which is thelight-emitting layer 3, is formed at an oxygen pressure of 700 mTorr atsubstrate temperatures of 700° C. Then, a perovskite oxide SrTiO₃ thinfilm, which is the hole transport layer 4, is formed at an oxygenpressure of 700 mTorr at substrate temperatures of 700° C. tomanufacture a multilayer structure of the electron transport layer 2/thelight-emitting layer 3/the hole transport layer 4 on the substrate(lower electrode) 1.

Then, a CeO₂ film is continuously formed as the buffer layer 5 at anoxygen pressure of 700 mTorr at substrate temperatures of 700° C. It hasbeen experimentally clarified that the indium atoms of the upperelectrode 6 comprising an ITO film cause the luminance degradation ofthe light-emitting layer 3. This CeO₂ buffer layer 5 serves as thebuffer layer 5 for completely separating the indium and thelight-emitting layer 3. At this time, the oxygen pressure was varied inthe range of 10 mTorr or more and 700 mTorr or less, but similar resultswere obtained.

When the ITO film of the transparent upper electrode 6 is formed withoutperforming heat treatment, and an alternating voltage is applied, it isnoted that dielectric breakdown occurs before the start of lightemission, and light emission characteristics are not confirmed.Therefore, atmospheric heat treatment is performed in oxygen in therange of 900° C. or more and 1200° C. or less to improve the withstandvoltage of the light-emitting layer 3 to enable light emission, andthen, a transparent electrically conductive film, ITO, is formed on topto provide the upper electrode 6.

In the manufacturing of the perovskite oxide thin film EL element of thepresent invention, film formation was performed at each of substratetemperatures of 600° C., 700° C., and 800° C. In order to examine thecrystal structure, x-ray diffraction was measured. As a result, it wasconfirmed that the multilayer structure was epitaxially grown in the(001) direction at all temperatures. As a typical example, an x-raydiffraction pattern during growth at 700° C. is shown in FIG. 2.

FIG. 2 is an intensity-2θ characteristic diagram in which a1 is acomposition of Nb-STO(100)/CSTO:Pr(100)/STO(100), b1 is a composition ofNb-STO(200)/CSTO:Pr(200)/STO(200), and c1 is a composition ofNb-STO(300)/CSTO:Pr(300)/STO(300). The angle between the incident X-rayand the sample surface is θ, and the angle between the direction ofincidence and the direction of reflection is 2θ.

It can be seen that the X-ray diffraction pattern of the thin filmsappears only in the (001) direction. As a result, it can be seen thatthe thin films are epitaxially grown in the (001) direction.

Furthermore, in the manufacturing of the perovskite oxide thin film ELelement of the present invention, heat treatment was performed on thethin films formed at each of substrate temperatures of 600° C., 700° C.,and 800° C., then the upper electrode (transparent electrode) wasformed, and the EL characteristics were examined. As a typical example,EL characteristics measured for the sample formed at 700° C. andobtained at an alternating voltage of 25 V at a frequency of 1 kHz areshown in FIG. 3.

In the intensity-wavelength characteristics in FIG. 3, the peak value ofintensity is at a wavelength of 612 nm. Its background is confirmed at awavelength of 580 nm and a wavelength of 640 nm.

The light emission start voltage was in the range of 5 V to 20 V. Atabout 50 V, dielectric breakdown occurred, and light emission stopped.It was confirmed that light emission occurred in the entire ITO thinfilm electrode pad formed as the upper electrode (transparentelectrode), and it was recognized that the light emission mode wassurface light emission. As shown in FIG. 3, light emissioncharacteristics are obtained at a wavelength of 612 nm, and it isunderstood that the light emission characteristics are red. The lightemission start voltage was 5 V to 20 V as described above, and lightemission was confirmed in the range of from this light emission startvoltage to about 50 V, which was the dielectric breakdown voltage. Itwas confirmed that this light emission voltage was low voltage drive,compared with the fact that the light emission start voltage with othersulfides and oxides was 200 V or more. Non-Patent Document 7 describesthat red fluorescence characteristics are obtained with((Ca_(1-x)Sr_(x))_(1-y)Pr_(y))TiO₃: 0≦x≦1 and 0.001≦y≦0.2. It isconfirmed that similar fluorescence characteristics are also obtainedwith ((Ca_(1-x)Sr_(x))_(1-y)Pr_(y))TiO₃: 0≦x≦1 and 0.001≦y≦0.2, sincethese results were obtained with the above stoichiometric compositionoptimal for the light-emitting material.

Furthermore, since a perovskite oxide epitaxial thin film EL element wassuccessfully obtained by using an oxide perovskite electricallyconductive 1% Nb-substituted SrTiO₃ single crystal substrate as thesubstrate material, epitaxial growth is possible even if a substratematerial and a perovskite-related electrically conductive thin filmmaterial having a lattice constant in the vicinity of 3.905 nm are used.A single substrate is a SrTiO₃ single crystal substrate havingconductivity with the amount of Nb added being 0.1% or more.Furthermore, it is possible to form a conductive thin film on asubstrate having no conductivity for use as the lower electrode. At thistime, examples of a perovskite-related, electrically conductivesubstrate material having no conductivity include SrTiO₃, LaAlO₃, MgO,LaGaO₃, PrGaO₃, NdGaO₃, and SrLaAlO₃. Even if a conductive thin film isformed thereon with SrRuO₃, YBaCuO₇, or 0.1% or more Nb-substitutedSrTiO₃ having a lattice constant in the vicinity of 3.905 nm, and an ELelement structure is formed thereon, epitaxial growth is achieved.Therefore, since an EL element having excellent crystallinity isobtained, similar characteristics are obtained.

In the invention according to this embodiment, a structure in which thelight-emitting layer 3 is sandwiched between the dielectric thin films2, 4 is used. The EL light emission mechanism is considered as follows.When an electric field is applied to a sandwich structure of a transportlayer/a light-emitting layer/a transport layer, electrons trapped at theinterface level on the negative electrode side are tunneled to theconduction band of the light-emitting layer. These electrons become“hot” in the process of being accelerated by the applied electric field,and these hot electrons excite localized light-emitting centers. Whilethe light-emitting centers emit light, the electrons losing energy reachthe interface on the positive electrode side where the electrons aretrapped at the interface level. By making the applied electric fieldalternating, these series of processes are repeated, and the lightemission is continued. Considering this EL light emission mechanism,since light emission is provided by an alternating-current power supply,light emission occurs when at least one dielectric is laminated. Inrecent study, fluorescence is confirmed by adding a rare earth element,for example, one or more elements of Pr, Eu, Tb, Dy, and Tm, to aperovskite oxide polycrystal. The principle of this fluorescence isconsidered as follows. When a fluorescent material is irradiated withultraviolet rays, rare earth atom ions in the crystal are excited. Theexcited ions fall into the ground state. The energy released at thistime becomes light, leading to light emission. In this manner, in theprinciples of fluorescence and EL, means for exciting rare earth atomions are externally irradiated light and an electric field respectively,and the principles are totally different from each other. A materialexhibiting a fluorescence phenomenon in principle does not alwaysexhibit EL.

A second embodiment of the present invention will be described, usingFIGS. 4 to 6.

FIG. 4 is a diagram showing the configuration of a perovskite oxide thinfilm EL element in which a multilayer film of a transport layer 8/alight-emitting layer 9/a transport layer 10/a light-emitting layer 11/atransport layer 12 comprising a thin film is formed on a substrate(lower electrode) 7, and a transparent electrode (upper electrode) 14 isformed thereon via a buffer layer 13, according to the invention in thisembodiment. The substrate (lower electrode) 7, the multilayer structureof the transport layer 8/the light-emitting layer 9/the transport layer10/the light-emitting layer 11/the transport layer 12, the buffer layer13, and the transparent electrode (upper electrode) 14 are alltransparent.

In FIG. 4, reference numeral 7 denotes a Nb-substituted STO(100)substrate; reference numeral 8 denotes a transport layer, an epitaxialfilm, SrTiO₃; reference numeral 9 denotes a light-emitting layer, anepitaxial film, CaSrTiO₃:Pr; reference numeral 10 denotes a transportlayer, an epitaxial film, SrTiO₃; reference numeral 11 denotes alight-emitting layer, an epitaxial film, CaSrTiO₃:Pr; reference numeral12 denotes a transport layer, an epitaxial film, SrTiO₃; referencenumeral 13 denotes a buffer layer, an epitaxial film, CeO₂; andreference numeral 14 denotes a transparent electrode, ITO.

The pulse laser deposition method is also used in the manufacturing ofthe perovskite oxide thin film EL element of the present invention. As atypical example, a polished electrically conductive 1% Nb-substitutedSrTiO₃(001) single crystal substrate is used for the substrate (lowerelectrode) 7. A perovskite oxide SrTiO₃ thin film, which is thetransport layer 8, is formed on the substrate 7 at an oxygen pressure of700 mTorr at substrate temperatures of 700° C. Then, a perovskite oxideCa_(0.6)Sr_(0.4)TiO₃:0.2% Pr thin film, which is the light-emittinglayer 9, is formed at an oxygen pressure of 700 mTorr at substratetemperatures of 700° C. Then, a perovskite oxide SrTiO₃ thin film, whichis the transport layer 10, is formed at an oxygen pressure of 700 mTorrat substrate temperatures of 700° C. Then, a perovskite oxideCa_(0.6)Sr_(0.4)TiO₃:0.2% Pr thin film, which is the light-emittinglayer 11, is continuously formed at an oxygen pressure of 700 Torr atsubstrate temperatures of 700° C. Then, a perovskite oxide SrTiO₃ thinfilm, which is the transport layer 12, is formed at an oxygen pressureof 700 mTorr at substrate temperatures of 700° C. Thus, a multilayerstructure with five layers of the transport layer 8/the light-emittinglayer 9/the transport layer 10/the light-emitting layer 11/the transportlayer 12 is formed on the substrate (lower electrode) 7. At this time,the oxygen pressure was varied in the range of 10 mTorr or more and 700mTorr or less, but similar results were obtained.

Then, a CeO₂ film is continuously formed as the buffer layer 13 at anoxygen pressure of 700 mTorr at substrate temperatures of 700° C. It hasbeen experimentally clarified that the indium atoms of the ITO film ofthe upper electrode 14 cause the luminance degradation of thelight-emitting layers 9, 11. This CeO₂ buffer layer 13 serves as thebuffer layer 13 for completely separating the indium and thelight-emitting layer 11.

When the ITO film of the transparent electrode (upper electrode) 14 isformed without performing heat treatment here, and an alternatingvoltage is applied, dielectric breakdown occurs before the start oflight emission, and light emission characteristics are not confirmed.Therefore, atmospheric heat treatment is performed in oxygen in therange of 900° C. or more and 1200° C. or less to improve the withstandvoltage of the light-emitting layers 9, 11 to enable light emission, andthen, a transparent electrically conductive film, ITO, is formed on topto provide the transparent electrode (upper electrode) 14.

In the manufacturing of the perovskite oxide thin film EL element of thepresent invention, film formation was performed at each of substratetemperatures of 600° C., 700° C., and 800° C. In order to examine thecrystal structure, x-ray diffraction was measured. As a result, it wasconfirmed that the multilayer structure was epitaxially grown in the(001) direction at all temperatures. As a typical example, an x-raydiffraction pattern during growth at 700° C. is shown in FIG. 5.

FIG. 5 is an intensity-2θ characteristic diagram in which a2 is acomposition of Nb-STO(100)/CSTO:Pr(100)/STO(100), b2 is a composition ofNb-STO(200)/CSTO:Pr(200)/STO(200), and c2 is a composition ofNb-STO(300)/CSTO:Pr(300)/STO(300).

The pattern of the thin films appears only in the (001) direction.Therefore, it is confirmed that the thin films are epitaxially grown inthe (001) direction.

Furthermore, in the manufacturing of the perovskite oxide thin film ELelement of the present invention, heat treatment was performed on thethin films formed at each of substrate temperatures of 600° C., 700° C.,and 800° C., the transparent electrode (upper electrode) was formed, andthe EL characteristics were examined. As a typical example, ELcharacteristics measured for the sample formed at 700° C. and obtainedat an alternating voltage of 40 V at a frequency of 1 kHz are shown inFIG. 6.

In the intensity-wavelength characteristics in FIG. 6, the peak value ofintensity is at a wavelength of 612 nm. Its background is confirmed at awavelength of 580 nm and a wavelength of 640 nm.

The light emission start voltage was in the range of 10 V to 15 V, andlight emission was achieved in the range of up to about 50 V. It wasconfirmed that light emission occurred in the entire ITO thin filmelectrode pad formed as the transparent electrode (upper electrode), andit was confirmed that the light emission mode was surface lightemission. In FIG. 6, light emission characteristics are obtained at awavelength of 612 nm, and it is understood that the light emissioncharacteristics are red. The light emission start voltage was asdescribed above, and therefore, low voltage drive was confirmed. This isconsidered to be due to the fact that the insulating thin films have ahigh dielectric constant value because of excellent crystallinity.Non-Patent Document 7 describes that red fluorescence characteristicsare obtained with ((Ca_(1-x)Sr_(x))_(1-y)Pr_(y))TiO₃: 0≦x≦1 and0.001≦y≦0.2. It is confirmed that similar fluorescence characteristicsare also obtained with ((Ca_(1-x)Sr_(x))_(1-y)Pr_(y))TiO₃: 0≦x≦1 and0.001≦y≦0.2, since these results were obtained with the abovestoichiometric composition optimal for the light-emitting material.

A third embodiment of the present invention will be described, usingFIGS. 7 to 9.

FIG. 7 is a diagram showing the configuration of a perovskite oxide thinfilm EL element in which a multilayer film of a lower electrode 16/anelectron transport layer 17/a light-emitting layer 18/a hole transportlayer 19 comprising a thin film is formed on a substrate 15, and atransparent electrode (upper electrode) 21 is formed thereon via abuffer layer 20, according to the invention in this embodiment. Thesubstrate 15, the multilayer structure of the lower electrode 16/theelectron transport layer 17/the light-emitting layer 18/the holetransport layer 19, the buffer layer 20, and the transparent electrode(upper electrode) 21 are all transparent.

In FIG. 7, reference numeral 15 denotes an STO(100) substrate; referencenumeral 16 denotes an electrode, an epitaxial film, 1% Nb-substitutedSrTiO₃; reference numeral 17 denotes an electron transport layer, anepitaxial film, SrTiO₃; reference numeral 18 denotes a light-emittinglayer, an epitaxial film, CaSrTiO₃:Pr; reference numeral 19 denotes ahole transport layer, an epitaxial film, SrTiO₃; reference numeral 20denotes a buffer layer, an epitaxial film, CeO₂; and reference numeral21 denotes a transparent electrode, ITO:

An electrically conductive 1% Nb-substituted SrTiO₃ substrate isexpensive as the material of the substrate 15. Therefore, a case will bedescribed where SrTiO₃(100), which is more inexpensive than theelectrically conductive 1% Nb-substituted SrTiO₃ substrate, is used asthe material of the substrate 15, and a perovskite oxide thin film ELelement is made on this substrate 15. As a typical example, an one sidepolished semitransparent-SrTiO₃(001) single crystal substrate having noelectrical conductivity is used for the substrate 15. The crystalstructure of the substrate 15 is tetragonal. The substrate 15 has alattice constant of 3.905 nm, has no electrical conductivity, and isused as the lower substrate material. Many perovskite oxide materialshave a lattice constant in the vicinity of the numerical value of theabove lattice constant and have good lattice matching properties withthe substrate 15. Therefore, it is possible to grow an oxide epitaxialthin film having excellent crystallinity grown on the substrate 15.Other than the polished SrTiO₃(001) single crystal substrate, there arepolished LaAlO₃, MgO, LaGaO₃, PrGaO₃, NdGaO₃, and SrLaAlO₃ singlecrystal substrates.

First, a 1% Nb-substituted SrTiO₃ thin film is formed on the substrate15 at an oxygen pressure of 700 mTorr at substrate temperatures of 700°C. to form the lower electrode 16. Next, a perovskite oxide SrTiO₃ thinfilm, which is the electron transport layer 17, is formed at an oxygenpressure of 700 mTorr at substrate temperatures of 700° C. Then, aperovskite oxide Ca_(0.6)Sr_(0.4)TiO₃:0.2% Pr thin film, which is thelight-emitting layer 18, is formed at an oxygen pressure of 700 mTorr atsubstrate temperatures of 700° C. Then, a perovskite oxide SrTiO₃ thinfilm, which is the hole transport layer 19, is formed at an oxygenpressure of 700 mTorr at substrate temperatures of 700° C. tomanufacture a multilayer structure of the electron transport layer17/the light-emitting layer 18/the hole transport layer 19 on the lowerelectrode 16.

Then, a CeO₂ film is continuously formed as the buffer layer 20 at anoxygen pressure of 700 mTorr at substrate temperatures of 700° C. It hasbeen experimentally clarified that the indium atoms of an ITO film,which is the transparent upper electrode 21, cause the luminancedegradation of the light-emitting layer 18. Therefore, this CeO₂ bufferlayer 20 serves as the buffer layer 20 for completely separating theindium and the light-emitting layer 18.

When the ITO film, which is the transparent electrode (upper electrode)21, is formed without performing heat treatment here, and an alternatingvoltage is applied, dielectric breakdown occurs before the start oflight emission, and light emission characteristics are not confirmed.Therefore, atmospheric heat treatment is performed in oxygen in therange of 900° C. or more and 1200° C. or less to improve the withstandvoltage of the light-emitting layer 18 to enable light emission, andthen, a transparent electrically conductive film, ITO, is formed on topto provide the transparent electrode (upper electrode) 21. The lowerelectrode 16 is blue close to black because Nb-substituted SrTiO₃ isused, and the multilayer structure of the electron transport layer17/the light-emitting layer 18/the hole transport layer 19 is alltransparent.

In the manufacturing of the perovskite oxide thin film EL element of thepresent invention, film formation was performed at each of substratetemperatures of 600° C., 700° C., and 800° C. In order to examine thecrystal structure, x-ray diffraction was measured. As a result, it wasconfirmed that the multilayer structure was epitaxially grown in the(001) direction at all temperatures. As a typical example, an x-raydiffraction pattern during growth at 700° C. is shown in FIG. 8.

FIG. 8 is an intensity-2θ characteristic diagram in which a3 is acomposition of Nb-STO(100)/CSTO:Pr(100)/STO(100), b3 is a composition ofNb-STO(200)/CSTO:Pr(200)/STO(200), and c3 is a composition ofNb-STO(300)/CSTO:Pr(300)/STO(300).

The pattern of the thin films appears only in the (001) direction.Therefore, it is confirmed that all thin films are epitaxially grown inthe (001) direction.

Furthermore, in the manufacturing of the perovskite oxide thin film ELelement of the present invention, heat treatment was performed on thethin films formed at each of substrate temperatures of 600° C., 700° C.,and 800° C., the transparent electrode (upper electrode) was formed, andthe EL characteristics were examined. As a typical example, ELcharacteristics measured for the sample formed at 700° C. and obtainedat an alternating voltage of 25 V at a frequency of 1 kHz are shown inFIG. 9.

In the intensity-wavelength characteristics in FIG. 9, the peak value ofintensity is at a wavelength of 612 nm. Its background is confirmed at awavelength of 580 nm and a wavelength of 640 nm.

The light emission start voltage was in the range of 5 V to 20 V, andlight emission was achieved in the range of up to about 50 V. It wasconfirmed that light emission occurred in the entire ITO thin filmelectrode pad formed as the transparent electrode (upper electrode), andit was confirmed that the light emission mode was surface lightemission. In FIG. 9, light emission characteristics are obtained at awavelength of 612 nm, and it is understood that the light emissioncharacteristics are red. The light emission start voltage was asdescribed above, and therefore, low voltage drive was confirmed.Non-Patent Document 7 describes that red fluorescence characteristicsare obtained with ((Ca_(1-x)Sr_(x))_(1-y)Pr_(y))TiO₃: 0≦x≦1 and0.001≦y≦0.2. As a result, it is confirmed that similar fluorescencecharacteristics are also obtained with((Ca_(1-x)Sr_(x))_(1-y)Pr_(y))TiO₃: 0≦x≦1 and 0.001≦y≦0.2, since theseresults were obtained with the above stoichiometric composition optimalfor the light-emitting material.

Next, a fourth embodiment will be described, using FIGS. 10 to 12.

FIG. 10 is a diagram showing the configuration of a perovskite oxidethin film EL element in which a multilayer film of an electron transportlayer 23/a light-emitting layer 24/a hole transport layer 25 comprisinga thin film is formed on a substrate (lower electrode) 22, and atransparent electrode (upper electrode) 27 is formed thereon via abuffer layer 26, according to the invention in this embodiment.

In FIG. 10, reference numeral 22 denotes a Nb-substituted STO(100)substrate; reference numeral 23 denotes an electron transport layer, anepitaxial film, BaTiO₃; reference numeral 24 denotes a light-emittinglayer, an epitaxial film, CaSrTiO₃:Pr; reference numeral 25 denotes ahole transport layer, an epitaxial film, BaTiO₃; reference numeral 26denotes a buffer layer, an epitaxial film, CeO₂; and reference numeral27 denotes a transparent electrode, ITO.

In the perovskite oxide thin film EL element, it is expected that thedrive voltage is decreased by the relative dielectric constant of thedielectric material used for the transport layers. The decrease of thedrive voltage leads to the miniaturization of a drive power supply whena display is made using the perovskite oxide thin film EL element.Therefore, it is possible to decrease the weight and shape of the panelitself. The invention in this embodiment is characterized by usingBaTiO₃ as the transport layers 23, 25.

As a typical example, a polished electrically conductive 1%Nb-substituted SrTiO₃(001) single crystal substrate is used for thesubstrate (lower electrode) 22. First, a perovskite oxide BaTiO₃ thinfilm, which is the electron transport layer 23, is formed on thesubstrate (lower electrode) 22 at an oxygen pressure of 700 mTorr atsubstrate temperatures of 700° C. Then, a perovskite oxideCa_(0.6)Sr_(0.4)TiO₃:0.2% Pr thin film, which is the light-emittinglayer 24, is formed at an oxygen pressure of 700 mTorr at substratetemperatures of 700° C. Then, a perovskite oxide SrTiO₃ thin film, whichis the hole transport layer 25, is formed at an oxygen pressure of 700mTorr at substrate temperatures of 700° C. to manufacture a multilayerstructure of the electron transport layer 23/the light-emitting layer24/the hole transport layer 25 on the substrate (lower electrode) 22.

Then, a CeO₂ film is continuously formed as the buffer layer 26 at anoxygen pressure of 700 mTorr at substrate temperatures of 700° C. It hasbeen experimentally clarified that the indium atoms of an ITO film,which is the transparent electrode (upper electrode) 27, cause theluminance degradation of the light-emitting layer 24. Therefore, theCeO₂ buffer layer 26 for completely separating the indium and thelight-emitting layer 24 is provided.

When the ITO film of the transparent electrode (upper electrode) 27 isformed without performing heat treatment here, and an alternatingvoltage is applied, dielectric breakdown occurs before the start oflight emission, and light emission characteristics are not confirmed.Therefore, atmospheric heat treatment is performed in oxygen in therange of 900° C. or more and 1200° C. or less to improve the withstandvoltage of the light-emitting layer 24 to enable light emission, andthen, a transparent electrically conductive film, ITO, is formed on topfor use as the transparent electrode (upper electrode) 27. The substrate(lower electrode) 22, the multilayer structure of the electron transportlayer 23/the light-emitting layer 24/the hole transport layer 25, thebuffer layer 26, and the transparent electrode (upper electrode) 27 areall transparently formed.

In the manufacturing of the perovskite oxide thin film EL element of thepresent invention, film formation was performed at each of substratetemperatures of 600° C., 700° C., and 800° C. In order to examine thecrystal structure, x-ray diffraction was measured. As a result, it wasconfirmed that the multilayer structure was epitaxially grown in the(001) direction at all temperatures. As a typical example, an x-raydiffraction pattern during growth at 700° C. is shown in FIG. 11.

FIG. 11 is an intensity 2θ characteristic diagram in which a4 is acomposition of Nb-STO(100)/CSTO:Pr(100)/STO(100), b4 is a composition ofNb-STO(200)/CSTO:Pr(200)/STO(200), and c4 is a composition ofNb-STO(300)/CSTO:Pr(300)/STO(300).

The pattern of the thin films appears only in the (001) direction.Therefore, it is confirmed that the thin films are epitaxially grown inthe (001) direction.

Furthermore, in the manufacturing of the perovskite oxide thin film ELelement of the present invention, heat treatment was performed on thethin films formed at each of substrate temperatures of 600° C., 700° C.,and 800° C., the transparent electrode (upper electrode) was formed, andthe EL characteristics were examined. As a typical example, ELcharacteristics measured for the sample formed at 700° C. and obtainedat an alternating voltage of 10 V at a frequency of 1 kHz are shown inFIG. 12.

In the intensity-wavelength characteristics in FIG. 12, the peak valueof intensity is at a wavelength of 612 nm. Its background is confirmedat a wavelength of 580 nm and a wavelength of 640 nm.

The light emission start voltage was in the range of 5 V to 8 V, andlight emission was achieved in the range of up to about 40 V. It wasconfirmed that light emission occurred in the entire ITO thin filmelectrode pad formed as the transparent electrode (upper electrode), andit was confirmed that the light emission mode was surface lightemission. In FIG. 12, light emission characteristics are obtained at awavelength of 612 nm, and it is understood that the light emissioncharacteristics are red. The light emission start voltage was asdescribed above, and therefore, low voltage drive was confirmed.Non-Patent Document 7 describes that red fluorescence characteristicsare obtained with ((Ca_(1-x)Sr_(x))_(1-y)Pr_(y))TiO₃: 0≦x≦1 and0.001≦y≦0.2. As a result, it is confirmed that similar fluorescencecharacteristics are also obtained with((Ca_(1-x)Sr_(x))_(1-y)Pr_(y))TiO₃: 0≦x≦1 and 0.001≦y≦0.2, since theseresults were obtained with the above stoichiometric composition optimalfor the light-emitting material.

In the perovskite oxide thin film EL element of the present invention,light emission occurs by applying an alternating voltage, and lightemission occurs at both the positive and negative poles of thealternating voltage by sandwiching both sides of the light-emittinglayer 24 between the transport layers 23, 25, which are dielectrics. Inthe case of only one side, light emission occurs with one of eitherpositive or negative polarity. Therefore, EL occurs even if thetransport layer 23 or the transport layer 25, which is a dielectric, isformed on either one side of the light-emitting layer 24.

Furthermore, in the inventions in the above embodiments, the latticeconstant of the perovskite oxide materials is mainly about 0.39 nm, anda material having a lattice constant in the vicinity of this is used asthe light-emitting layer. When a multilayer structure is made withepitaxial thin films, epitaxial growth is performed with a latticemismatch within ±8%. Therefore, the above EL characteristics areobtained also when a dielectric having a lattice constant in the rangeof 0.39 nm±0.03 nm is used as the transport layer.

Furthermore, in the inventions in the above embodiments, CeO₂ was usedas the buffer layer in all structures to prevent the indium atoms of theITO film from degrading the light-emitting layer. When, experimentally,CeO₂ was not formed, the lifetime of light emission was within 1 minute.It was confirmed that the lifetime of the light-emitting layer wassignificantly improved by the formation of this CeO₂ layer. Further,CeO₂ was used as the buffer layer, and the ITO film was used as thetransparent electrode, but even if this portion was changed to a SnO₂transparent electrode, light emission was obtained.

The lattice arrangement and surface structure of the thin films formedin this application, and crystal lattice arrangement at the boundarysurfaces between different types of materials will be described below.The result of the X-ray φ scan of CSTO:Pr(100), which is thelight-emitting layer obtained in the present invention, is shown in FIG.13. A diffraction peak appears for each 90.0 degrees.

Phi (deg.) is an angle of rotation around one normal perpendicular to asample surface. As a result, it is confirmed that (010) and (001) arearranged in a plane. In other words, it is clear from FIG. 2, FIG. 5,FIG. 8, and FIG. 11 that (100) is a direction perpendicular to theplane, and it is confirmed that in the CSTO:Pr(100) crystal formed inthis application, three crystal axes are oriented. It is also confirmedthat the SrTiO₃ thin film used for the transport layer in thisapplication, having a lattice constant similar to that of thislight-emitting layer, has the lattice arrangement similar to that of theabove CSTO:Pr(100).

Next, the result of the AFM observation of the light-emitting layerobtained in the present invention is shown in FIG. 14.

FIG. 14 shows the flatness of an as-grown film surface immediately afterfilm formation (see FIG. 14( a)) and flatness after heat treatment (seeFIG. 14( b)). The characteristics in the lower part (see FIG. 14( c))are the result of line scan for the flatness after heat treatment.

The horizontal axes in FIGS. 14( a), 14(b) are the x-axis in the samplesurface and indicate a maximum of 1 (μm). The vertical axes are they-axis and indicate a maximum of 1 (μm). The horizontal axis in FIG. 14(c) is the x-axis with a maximum of 1 (μm). The vertical axis is thez-axis with a maximum of 1.41 (nm) and represents the degree ofunevenness of the surface.

In the characteristics in FIG. 14( c), there are four terraces, and thedifference in level between them is 0.39 nm, which is equal to thelattice constant of (Sr_(0.4)Ca_(0.6))TiO₃:Pr. It is confirmed that theboundary between the light-emitting layer and the transport layer of theEL element is flat at the atomic level. Next, whether oriented growthoccurred continuously in the vicinity of the boundary between thelight-emitting layer and the insulator was observed by cross-sectionTEM.

The results are described using FIGS. 15, 16.

FIG. 15 is the result of observation of the vicinity of the boundarybetween the lower transport layer and the light-emitting layer of thetransport layer/the light-emitting layer/the transport layer on thesingle crystal substrate. It is confirmed that the crystal lattice iscontinuously arranged above and below the boundary.

FIG. 16 is the result of observation of the vicinity of the boundarybetween the light-emitting layer and the upper transport layer of thetransport layer/the light-emitting layer/the transport layer on thesingle crystal substrate. It is confirmed that the crystal lattice iscontinuously arranged above and below the boundary.

From these results, it is confirmed that multiple oriented growth occurscontinuously at the boundaries having a normal interface with flatnesson the atomic order.

This application is characterized in that multiple oriented growthoccurs continuously near the boundaries between different materials. Asa result, a suitable state density distribution is obtained in thevicinity of the boundary between the transport layer comprising theinsulator and the light-emitting layer. Carriers are produced duringelectric field application, and the carriers are accelerated by avoltage applied to the light-emitting layer. The accelerated carrierscollide with rare earth atom-ions, which are light-emitting centers,contribute energy, and are excited, leading to light emission. Thislight emission process is largely different from the process offluorescence, and it is not meant that materials with fluorescence allexhibit EL.

1. An oriented perovskite oxide thin film EL element comprising: a lowerelectrode comprising a polished single crystal substrate, a firsttransport epitaxial layer comprising an oriented perovskite oxide thinfilm, which is a dielectric, formed on the single crystal substrate, alight-emitting epitaxial layer comprising an oriented perovskite oxidethin film formed on the first transport epitaxial layer, a secondtransport epitaxial layer comprising an oriented perovskite oxide thinfilm, which is a dielectric, formed on the light-emitting epitaxiallayer, a buffer layer formed on the second transport epitaxial layer,and a transparent upper electrode formed on the buffer layer.
 2. Theoriented perovskite oxide thin film EL element according to claim 1,wherein each of the first and second transport epitaxial layerscomprising perovskite oxide, which is a dielectric, has a latticeconstant in the range of 0.39 nm±0.03 nm.
 3. The oriented perovskiteoxide thin film EL element according to claim 1, wherein the transparentupper electrode comprises an ITO film, and wherein the buffer layerinhibits a luminance degradation of the light-emitting epitaxial layerby indium atoms.
 4. An oriented perovskite oxide thin film EL elementcomprising: a lower electrode comprising a polished single crystalsubstrate, a first transport epitaxial layer comprising an orientedperovskite oxide thin film, which is a dielectric, formed on the singlecrystal substrate, a first light-emitting epitaxial layer comprising anoriented perovskite oxide thin film formed on the first transportepitaxial layer, a second transport epitaxial layer comprising anoriented perovskite oxide thin film, which is a dielectric, formed onthe first light-emitting epitaxial layer, a second light-emittingepitaxial layer comprising an oriented perovskite oxide thin film formedon the first transport epitaxial layer formed on the second transportlayer, a third transport epitaxial layer comprising an orientedperovskite oxide thin film, which is a dielectric, formed on the secondlight-emitting epitaxial layer, a buffer layer formed on the thirdtransport epitaxial layer, and a transparent upper electrode formed onthe buffer layer.
 5. The oriented perovskite oxide thin film EL elementaccording to claim 4, wherein the transparent upper electrode comprisesan ITO film, and wherein the buffer layer inhibits a luminancedegradation of the light-emitting epitaxial layer by indium atoms.
 6. Anoriented perovskite oxide thin film EL element for emitting red lightcomprising: a lower electrode comprising a polished single crystalsubstrate of SrTiO₃(001) in which 0.1% or more of Ti is substituted withNb, a first transport epitaxial layer comprising an oriented thin filmof perovskite oxide SrTiO₃, which is a dielectric, formed on the singlecrystal substrate, a light-emitting epitaxial layer comprising anoriented thin film of perovskite oxide((Ca_(1-x)Sr_(x))_(1-y)Pr_(y))TiO₃: 0≦x≦1 and 0.001≦y≦0.2 formed on thefirst transport epitaxial layer, a second transport epitaxial layercomprising an oriented thin film of perovskite oxide SrTiO₃, which is adielectric, formed on the light-emitting epitaxial layer, a CeO₂ filmbuffer layer formed on the second transport epitaxial layer, and atransparent upper electrode comprising an ITO film formed on the bufferlayer.
 7. An oriented perovskite oxide thin film EL element for emittingred light comprising: a lower electrode comprising a polished singlecrystal substrate of SrTiO₃(001) in which 0.1% or more of Ti issubstituted with Nb, a first transport epitaxial layer comprising anoriented thin film of perovskite oxide SrTiO₃, which is a dielectric,formed on the single crystal substrate, a first light-emitting epitaxiallayer comprising an oriented thin film of perovskite oxide((Ca_(1-x)Sr_(x))_(1-y)Pr_(y))TiO₃: 0≦x≦1 and 0.001≦y≦0.2 formed on thefirst transport epitaxial layer, a second transport epitaxial layercomprising an oriented thin film of perovskite oxide SrTiO₃, which is adielectric, formed on the first light-emitting epitaxial layer, a secondlight-emitting epitaxial layer comprising an oriented thin film ofperovskite oxide ((Ca_(1-x)Sr_(x))_(1-y)Pr_(y))TiO₃: 0≦x≦1 and0.001≦y≦0.2 formed on the second transport epitaxial layer, a thirdtransport epitaxial layer comprising an oriented thin film of perovskiteoxide SrTiO₃, which is a dielectric, formed on the second light-emittingepitaxial layer, a CeO₂ film buffer layer formed on the third transportepitaxial layer, and a transparent upper electrode comprising an ITOfilm formed on the buffer layer.
 8. An oriented perovskite oxide thinfilm EL element for emitting red light comprising: a polished singlecrystal substrate of SrTiO₃(001), a lower electrode comprising anoriented thin epitaxial film of SrTiO₃ in which 0.1% or more of Ti issubstituted with Nb, formed on the single crystal substrate, a firsttransport epitaxial layer comprising an oriented thin film of perovskiteoxide SrTiO₃, which is a dielectric, formed on the lower electrode, alight-emitting epitaxial layer comprising an oriented thin film ofperovskite oxide ((Ca_(1-x)Sr_(x))_(1-y)Pr_(y))TiO₃: 0≦x≦1 and0.001≦y≦0.2 formed on the first transport epitaxial layer, a secondtransport epitaxial layer comprising an oriented thin film of perovskiteoxide SrTiO₃, which is a dielectric, formed on the light-emittingepitaxial layer, a CeO₂ film buffer layer formed on the second transportepitaxial layer, and a transparent upper electrode comprising an ITOfilm formed on the buffer layer.
 9. An oriented perovskite oxide thinfilm EL element for emitting red light comprising: a lower electrodecomprising a polished single crystal substrate of SrTiO₃(001) in which0.1% or more of Ti is substituted with Nb, a first transport epitaxiallayer comprising an oriented thin film of perovskite oxide BaTiO₃, whichis a dielectric, formed on the single crystal substrate, alight-emitting epitaxial layer comprising an oriented thin film ofperovskite oxide ((Ca_(1-x)Sr_(x))_(1-y)Pr_(y))TiO₃: 0≦x≦1 and0.001≦y≦0.2 formed on the first transport epitaxial layer, a secondtransport epitaxial layer comprising an oriented thin film of perovskiteoxide BaTiO₃, which is a dielectric, formed on the light-emittingepitaxial layer, a CeO₂ film buffer layer formed on the second transportepitaxial layer, and a transparent upper electrode comprising an ITOfilm formed on the buffer layer.